﴿ وَقَالُوا۟ لَن نُّؤْمِنَ لَكَ حَتَّىٰ تَفْجُرَ لَنَا مِنَ ٱلْأَرْضِ يَنۢبُوعًا ﴾

“and so they say: "[O Muhammad,] we shall not believe thee till thou cause a spring to gush forth for us from the earth,”

And they say wa-qālū is a supplement to abā illā ‘insist on’ ‘We will not believe you until you make gush forth for us from the ground a spring from which water issues forth;